1. Field of the Invention
The present invention relates to a multilayer LC filter for use in mobile communication devices such as cellular phones.
2. Description of the Related Arts
Various small-sized LC filters for use in mobile communication devices such as cellular phones have hitherto been proposed which feature low losses and are capable of high mass-productivity. A circuit diagram for such a LC filter is shown in FIG. 5, where a multilayer LC filter Axe2x80x2 comprises an input/output line M connecting an input terminal 3 and an output terminal 4, with three capacitors C being connected in series in the input/output line M, and with another capacitor C being connected in the input/output line M in parallel with the above-mentioned capacitors. Two LC resonance circuits 1 and 2, in a two-stage circuit configuration, are connected across the three capacitors C with both of the circuits 1 and 2 being grounded. The LC resonance circuit 1 on the side of the input terminal 3 comprises a resonator capacitor C1 and a resonator inductor L1, and the LC resonator circuit 2 on the side of the output terminal 4 comprises a resonator capacitor C2 and a resonator inductor L2. In this configuration, the resonator capacitor C1 and the resonator inductor L1 are connected in parallel with each other, and the resonator capacitor C2 and the resonator inductor L2 are connected in parallel with each other, with the resonator inductors L1 and L2 of the LC resonance circuits 1 and 2 being coupled magnetically to each other.
Referring to FIG. 6, which is an exploded perspective view of the physical construction of the multilayer LC filter Axe2x80x2, filter Axe2x80x2 is a laminate which includes a plurality of conductive electrical paths 9, and electrodes 10, with dielectric layers disposed therebetween. More particularly, the multilayer LC filter Axe2x80x2 is preferably obtained by patterning and laminating predetermined conductive electrical paths 9 and electrodes 10 on dielectric sheets and by integrating all of them into a single-piece construction. Specifically, in this process, two strip-like electrical paths 5 and 6, constituting the resonator inductors L1 and L2 of FIG. 5, are patterned as strips extending in laterally spaced parallel relation to each other on the same dielectric layer. The ends of the strip-like or strip-shaped electrical paths 5 and 6 are respectively connected to electrical paths 9a and 9b, constituting the resonator capacitors C1 and C2 of FIG. 5, patterned on another dielectric layer. The other base ends of paths 5 and 6 are connected to a ground electrode 10a at the lowest position in the laminated structure that forms filter Axe2x80x2 through a strip-shaped lead or electrical path 7 which extends from an integrally-formed path portion 8 which extends orthogonally to, and is electrically connected to, these base ends.
The electrical paths for the resonator inductors, and other conductive components, in the above-discussed LC resonance circuit are generally patterned on the corresponding dielectric sheets by using a conventional screen printing technique or process. However, in using the screen printing process for patterning the electrical paths for the resonator inductors and other components, the effective lengths of the electrical paths for the resonator inductors may vary because of the process. Specifically, this variation can be caused by dispersion of the material used in the process. Further, the short-circuiting positions of the electrical paths may vary because of dislocations occurring in associated processes such as cutting. These problems are sometimes referred to as xe2x80x9cpattern dislocation.xe2x80x9d Such pattern dislocations cause a variation in the magnetic coupling characteristics between the resonator inductors disposed on the LC resonance circuit.
Referring to FIG. 7, examples of the effects of such pattern dislocation are presented. In FIG. 7, in a situation wherein the strip-like electrical path for the resonator inductor is dislocated by 30 xcexcm in the direction in which the strip-like electrical path extends toward the side where the resonator capacitors C1 and C2 are connected (hereinafter referred to as xe2x80x9c+ directionxe2x80x9d), the value of 2.5 dbBW (MHz) varies from 76.0 MHz to 98.0 MHz (a variation range of 22.0 MHz), the value of TOP L (db) varies from 2.27 db to 2.23 db (a variation range of xe2x88x920.04 db) and the value of R. L. (db) varies from 15.4 db to 12.7 db (a variation range of xe2x88x922.7 db). To briefly explain these values, 2.5 dbBW (MHz) is a value representing the bandwidth of the filter when the attenuation drops from 0 db to 2.5 db, while TOP L (db) is a value representing the minimum insertion loss in the pass band of the filter and R. L. is a value representing the value of the reflective attenuation in the pass band of the filter.
Such variation in the magnetic coupling characteristics of the multilayer LC filters during manufacture results in a variation in the quality of the multilayer LC filters and this leads to an increase in manufacturing costs where additional processing is needed for correction of the magnetic coupling characteristics.
The present invention is concerned with the problems discussed above and it is, therefore, an object of the present invention to provide a multilayer LC filter in which pattern dislocations which occur during patterning of the conductive elements or components thereof only minimally contribute to variations in the magnetic coupling characteristics of the associated LC resonance circuit or circuits.
According to a first aspect of the present invention, there is provided a multilayer LC filter comprising a first LC resonance circuit having a first resonator inductor; and a second LC resonance circuit having a second resonator inductor; the first resonator inductor and the second resonator inductor being comprised of two strip-shaped electrical paths, i.e., elongate electrically conductive elements, extending in laterally spaced parallel relation and patterned on the same dielectric layer, with the base end of each strip-shaped electrical path being connected to the ground, wherein a strip-shaped shielding ground electrical path is formed between, and disposed in parallel with, the first resonator inductor and the second resonator inductor.
The first resonator inductor and the second resonator inductor, comprised of the two strip-shaped electrical paths patterned in laterally spaced, parallel relation on the same dielectric layer, are magnetically coupled together. As indicated above, in this configuration, a shielding ground electrical path according to the invention is connected to ground and formed between, and disposed in parallel with, the resonator inductors. As a consequence, the amount of variation in magnetic coupling caused by pattern dislocation can be made smaller than that in a conventional structure which does not include the shielding ground electrical path.
For example, as shown in FIG. 3, when the strip-shaped electrical path of the resonator inductor is dislocated by 30 xcexcm in the + direction, the value of 2.5 dbBW (MHz) varies only by 5.0 MHz, the value of TOP L (db) varies only by xe2x88x920.02 db and the value of R. L. (db) varies only by xe2x88x921.0 db.
Comparing the aforementioned variation ranges for these magnetic field characteristics with that of a conventional configuration, as set forth in FIG. 7, it can be seen that the multilayer LC filter of the invention has a smaller variation range for each characteristic and has a reduced variation in the characteristics of the magnetic coupling relative to the pattern dislocations. For example, when the strip-shaped electrical path of the resonator inductor is dislocated by 30 xcexcm in the + direction, the variation range of the 2.5 dbBW characteristic of the invention is reduced by 17.0 MHz as compared with that of the conventional configuration.
By virtue of a configuration wherein an integrally-formed portion and a strip-shaped lead electrical path are also patterned on the same dielectric layer, and the integrally-formed portion is connected to and extends transversely with respect to the base ends of the shielding earth electrical path, the first resonator inductor and the second resonator inductor (all of which extend in parallel with each other), and the strip-shaped lead electrical path extends outwardly from the integrally-formed portion toward the opposite side of the dielectric layer away from the side where the resonator inductors are formed, the resonator inductors of the LC resonance circuits and the electrical paths for grounding the resonator inductors can all be formed on a single dielectric layer. It is thus possible to reduce the number of the layers or laminations of the multilayer LC filter and to thus meet the ever present need for a lower filter profile and a smaller filter size.